High-Pressure
Experimental and DFT‑D Structural
Studies of the Energetic Material FOX‑7
- Publication date
- Publisher
Abstract
This
work reports the hydrostatic compression of the perdeuterated
α-form of FOX-7 using neutron powder diffraction to follow the
structural changes up to 4.58 GPa at room temperature. The equation
of state for the hydrostatic compression of the α-form over
the range 0–4.14 GPa has been determined, and a phase transition
was observed over the pressure range 3.63–4.24 GPa. On the
basis of dispersion-corrected density functional theory (DFT-D) calculations
performed on the γ-form over a range of pressures, the high-pressure
form observed in the neutron diffraction experiments can unambiguously
be identified as being different from the γ-form and should
therefore be denoted as the ε-form. Based on similarities between
the simulated and experimental powder diffraction patterns of the
γ- and ε-forms, it is suggested that the ε-form
adopts a planar, layered structure. The structural responses to pressure
of the α-form observed experimentally are reproduced by DFT-D
calculations, but in-depth analysis of the bond lengths, angles, dihedrals,
and vibrational frequencies calculated in the DFT-D simulations identified
a very subtle second-order phase transition at 1.9 GPa. This corroborates
results obtained from previous far- and mid-IR vibrational spectroscopic
studies. These very small changes in molecular geometry do not manifest
themselves in either the measured or calculated lattice parameters
or unit-cell volumes and are much smaller than can be detected by
diffraction experiments. The results of phonon calculations were compared
with experimental inelastic neutron scattering measurements and were
used to investigate the effect of pressure on the heat capacities
of α-FOX-7. The simulations predict very weak pressure dependencies
(approximately −1 J K<sup>–1</sup> mol<sup>–1</sup> GPa<sup>–1</sup>), in accordance with the conclusions reached
in our previous studies of the energetic material RDX